Conductive adhesive and packaging structure using the same

ABSTRACT

The present invention provides a conductive adhesive and a packaging structure that can keep moisture-proof reliability even when a multipurpose base metal electrode is used. A conductive adhesive according to the present invention includes first particles having a standard electrode potential that is equal to or higher than a standard electrode potential of silver, and second particles having a standard electrode potential lower than a standard electrode potential of silver. A metal compound coating having a potential higher than that of metal particles as the first particles can be formed on a surface of an electrode having a potential lower than that of the metal particles.

This application is a divisional of application Ser. No. 09/898,721,filed Jul. 3, 2001, now U.S. Pat. No. 6,524,721 which application(s) areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a conductive adhesive used in a fieldof packaging of electronic elements. Furthermore, the present inventionrelates to a packaging structure including a substrate and an electronicelement that are electrically connected with each other via theconductive adhesive.

2. Description of the Prior Art

Due to a recent trend of environmentally-friendly sensitivity, controlsover lead included in solder alloys will be imposed in a field ofelectronic packaging, and thus, establishment of a lead-free packagingtechnique, i.e., a technique to join electronic elements with alead-free material is an urgent necessity. The lead-free packagingtechnique includes packaging using mainly a lead-free solder or alead-free conductive adhesive. Conductive adhesives have been notedparticularly in the technique since they are expected to provide meritssuch as joint flexibility and lower packaging temperatures.

A typical conductive adhesive is prepared by dispersing conductiveparticles in a resin-based adhesive ingredient (binder resin). Ingeneral, packaging of an element is carried out by applying a conductiveadhesive on a substrate electrode, packaging the element, andsubsequently curing the resin. In this way, the joints are adhered withthe resin, and the conductive particles are contacted with each otherdue to the contraction of the resin so that the conductivity at thejoints is secured. Since the curing temperature of the conductiveadhesive resin is about 150° C. and this is lower than a solder meltingpoint of 240° C., such a conductive adhesive can be used for inexpensiveparts having inferior heat resistance. Moreover, since the joints areadhered with a resin, they can respond flexibly to distortion caused byheat and/or external force. Therefore, the conductive adhesive has amerit that less cracks will occur at the joints when compared with asolder having alloy joints. For the above-mentioned reasons, aconductive adhesive is expected as an alternative to solder.

However, a conductive adhesive is inferior to a solder alloy in thepackaging reliability in a state being joined with a multipurposeelectrode element and with a substrate. In general, base metals such assolder alloys and Cu are used for terminal electrodes of circuit boardsand of electronic elements. When electronic elements and circuit boardshaving terminal electrodes of base metals are packaged with conductiveadhesives, the connection resistance is increased remarkably under anatmosphere with high temperature and high humidity. A major factorregarding the increasing connection resistance in a packaging structurewith a conductive adhesive is that the base metal used for theelectrodes corrodes in the presence of moisture. In other words,particles of a metal such as silver in the conductive adhesive contactwith moisture entering the base metal electrode so as to form a kind ofelectric cell, and thus, the base metal electrode having relatively lowpotential is corroded. Therefore, multipurpose cells should be replacedby electrodes of expensive metals such as Au or Pd in order to securemoisture-proof reliability when a conductive adhesive is used.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, the present inventionprovides a conductive adhesive and a packaging structure that canmaintain a moisture-proof reliability even when a multipurpose basemetal electrode is used.

A conductive adhesive according to the present invention is used toelectrically connect an electronic element and a substrate, and theconductive adhesive includes first particles having a standard electrodepotential equal to or higher than a standard electrode potential ofsilver and also second particles having a standard electrode potentiallower than a standard electrode potential of silver.

Since the conductive adhesive contains second particles having a lowpotential and the second particles are subject to sacrificial corrosion,corrosion in the electrodes of the electronic element and of thesubstrate electrode is controlled.

Furthermore, the present invention provides a packaging structureincluding an electronic element and a substrate in which the electronicelement and the substrate have electrodes connected electrically via theabove-mentioned conductive adhesive. In the packaging structure, thesecond particles corrode and the resulting ingredient can exist as atleast one kind of compound selected from an oxide, a hydroxide, achloride and a carbonate.

Moreover, the present invention provides a packaging structure includingan electronic element and a substrate, and the electronic element andthe substrate have electrodes connected electrically via a conductiveadhesive. This conductive adhesive includes particles having a standardelectrode potential that is equal to or higher than a standard electrodepotential of silver. A coating of a metal compound having a standardelectrode potential higher than that of the particles is formed on asurface of at least one electrode having a standard electrode potentiallower than that of the particles and composing the electronic element orthe substrate.

In the packaging structure, a coating of a metal compound having a highpotential is formed on a surface of the electrode having a low potentialin order to control corrosion in the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view to illustrate an embodiment of a packagingstructure in the present invention.

FIG. 2 is a plan view to illustrate a specimen used for evaluating aconductive adhesive in the present invention.

FIG. 3A is a perspective view to illustrate an example of a leadelement, and FIG. 3B is a cross-sectional view to illustrate anembodiment of a packaging structure using a lead according to thepresent invention.

FIGS. 4A and 4B show an analytic result for a composition in aconductive adhesive after a humidity test for samples provided accordingto the present invention, and the analysis is executed by using SIMS.FIG. 4A shows Zn distribution while FIG. 4B shows O distribution.

FIGS. 5A and 5B show analytic results for a vicinity of an interfacebetween a conductive adhesive and an electrode of an electronic elementafter a humidity test concerning a sample provided by a conventionaltechnique, and the analysis is executed by using SIMS. FIG. 5A shows Sndistribution while FIG. 5B shows O distribution.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments according to the present invention are describedbelow.

For achieving the above-mentioned purposes, a conductive adhesive usedin a first aspect of the present invention contains first particles,i.e., particles of a metal such as silver added to secure electricconductivity, and also second particles having a standard electrodepotential lower than that of the first particle. In a second aspect, anelectrode of the electronic element and/or of the substrate is subjectto surface treatment in order to raise the standard electrode potentialof the electrode.

The first aspect will be described as follows.

A conductive adhesive containing metal particles (first particles) tosecure electric connection, a binder resin, a curing agent and variousadditives is further provided with second particles. The secondparticles have a standard electrode potential lower than that of thefirst particles (i.e., the second particles are more easily corroded),and preferably, the standard electrode potential of the second particlesis even lower than those of the electronic element and the circuit boardto be connected with. In other words, a preferred relationship of thestandard electrode potentials is represented as follows:

(first particles)>(electrodes)>(second particles).

When the standard electrode potential of the second particles isrelatively lower than that of the electrodes, the electrodes can beprevented effectively from corroding, since the second particles areconnected electrically with the electrodes via the first particle so asto compose a corrosive cell and the second particles having relativelylow potential corrode prior to the electrodes. Specifically, thestandard electrode potential of the second particles is preferred to belower than that of Sn in view of the fact that a metal commonly used foran electrode surface is either Sn or an alloy containing Sn.

In a packaging structure, moisture that enters the joints becomes anelectrolyte to cause galvanic corrosion.

Actually, water-soluble ingredients contained in the conductive adhesiveor in the substrate can be dissolved in the entering water. Theingredients generate electrolytic ions that enhance the electrolyticproperty of the water and accelerate corrosion in the electrodes andsimultaneously, the ions accelerate sacrificial corrosion in the secondparticles. A certain amount of electrolytic ions will improvereliability in preventing corrosion. The inventors confirmed that 1 ppmto 10000 ppm of electrolytic ions are preferred to exist in a conductiveadhesive from a viewpoint of reliability improvement, and morepreferably, 1 ppm to 100 ppm. Preferable electrolytic ions includehalogen ions (especially, chloride ions) generated from bromine,chlorine or the like, and alkali metal ions generated from sodium,potassium or the like. In the examples described below, remarkableeffects for preventing corrosion were observed when Zn particles assecond particles and chloride ions coexisted.

Electrolytic ions may influence values of the standard electrodepotential. Therefore, when the conductive adhesive contains electrolyticions of 5 ppm or more, preferably, the above-described relationship isestablished even for a standard electrode potential where acommonly-used deionized water (the conductivity is not more than 1 μS⁻¹)is replaced by water containing electrolytic ions by taking an actualcorrosion process into consideration. The above-mentioned deionizedwater including 3 wt % NaCl can be used for a specific measuring object.

Preferably, the first particles are selected from particles of noblemetals such as gold (Au), silver (Ag), platinum (Pt), palladium (Pd),iridium (Ir), rhodium (Rh), osmium (Os), ruthenium (Ru), and alloys ofnoble metals such as an Ag-Pd alloy. Particles containing metals otherthan the noble metals, for example, copper (Cu) particles coated with Agalso can be used as long as the particles have a standard electrodepotential not lower than a standard electrode potential of silver.Silver (Ag) particles are preferred for the first particles when volumeresistivity values and material cost are taken into consideration.

A content of the first particles in the conductive adhesive isdetermined to keep the electric connection even when the secondparticles corrode. Specifically, the content may range from 70 wt % to95 wt % of the conductive adhesive.

The second particles are preferred to contain a base metal or anonmetal, specifically, at least one selected from iron (Fe), carbon(C), aluminum (Al), zinc (Zn), magnesium (Mg), nickel (Ni), copper (Cu),beryllium (Be), chromium (Cr), tin (Sn), vanadium (V) and calcium (Ca).

The second particles are preferred to have a property for easy formationof an oxide. Specifically, the inclusion of at least one selected fromZn, Fe, Mg, Cu, V, Ca and Be, and Zn is the most preferred. The secondparticles, when oxidized, contain on the surface many hydroxide groupsthat are easy to be bonded chemically with resin. Adherence between thebinder resin and the metal particles is improved due to the bonding, andthus, moisture can be prevented from entering the joints.

The second particles can contain plural elements such as carbon steel,SnAg, SnBi, SnCu, FeNi, BeCu and stainless steel. When an alloy is usedfor the second particles, an ingredient having a lower potential (Sn forSnAg) can be used to compare the potentials.

A content of the second particles in general may range from 0.5 wt % to10 wt % of the conductive adhesive. When the content is too low,sufficient effects cannot be obtained in controlling corrosion, whileexcessively high content may affect negatively the conductivity of theconductive adhesive. From this point of view, a content of the secondparticles is preferred to be higher than 2 wt %, more preferably notless than 3 wt % of the conductive adhesive.

When a lead element is used for the electronic element, effects inpreventing corrosion are improved by selecting the content of the secondparticles to be more than 2 wt % but not more than 10 wt %. The reasoncan be considered as follows. Compared to a case where a chip element isused, less pressure will be applied to a conductive adhesive at a timeof mounting when a lead element is used similar to a case of QFP (quadflat package). Unlike a chip element provided with a terminal electrodeto the device, a lead of a lead element is projected to make a terminalelectrode that functions as a spring to relax stress applied to thedevice.

When sufficient pressure is not applied to the conductive adhesive;sufficient electric contact cannot be kept between the first particles(e.g., Ag particles) and the electrode surface (e.g., SnPb). In thissituation, electric connection between the second particles (e.g., Znparticles) and the electrode can be insufficient and the secondparticles tend to be consumed due to self-corrosion. Therefore, when achip element is packaged, the amount of the second particles to be addedis preferred to increase slightly to make up for the self-corrosion.

Since addition of the second particles serves to control corrosion, abase metal can be used for the electrodes of the electronic element andof the circuit board. Though there is no specific limitation on basemetals used for the electrodes, corrosion control is especiallyeffective when the metal is at least one that is vulnerable to galvaniccorrosion and selected from Sn, Pb, Cu, Ni, Fe and Be.

When the thus obtained packaging structure where corrosion at the jointsis controlled is used or kept under a circumstance to accelerategalvanic corrosion, corrosion in the second particles will progress. Asa result, the second particles will be a corrosion product (typically atleast one compound selected from an oxide, a hydroxide, a chloride, anda carbonate). The present invention includes a packaging structurecomprising the second particles that have been corroded and modifiedfrom the initial state at the time of addition. In this embodiment, thecorrosion controlling effect of the second particles can be confirmed ifcorrosion progresses more in the second particles than in theelectrodes.

As mentioned above, the present invention can be regarded also as amethod of controlling corrosion of an electrode, and the method includesa step of preparing a conductive adhesive containing first particleshaving a standard electrode potential higher than a standard electrodepotential of silver and second particles having a standard electrodepotential lower than a standard electrode potential of silver; and astep of controlling corrosion of an electrode of the electronic elementand/or of the substrate by packaging the electronic element on thesubstrate via the conductive adhesive.

The conductive adhesive can include further an organic solvent. Sincethe added organic solvent serves to partially dissolve resin ingredientsat the interface between the conductive adhesive and the electrode, goodelectric contact between the first particles and the electrode can bemaintained. Glycol ethers or the like are preferably used to obtain thiseffect, and more specifically, the following ingredients can be used:diethylene glycol monobutyl ether, diethylene glycol monobutyl etheracetate, ethylene glycol monobutyl ether, ethylene glycol monobutylether acetate, propylene glycol monomethyl ether, propylene glycolmonomethyl ether acetate, propylene glycol monoethyl ether, andpropylene glycol monoethyl ether acetate.

Addition of an organic solvent having high polarity can improvemoisture-proof further qualities since such an organic solvent tends toact as a medium for a sacrificial corrosion reaction. To obtain thiseffect, it is preferable to use an organic solvent having a dielectricconstant of at least 15, for example, DEG (diethylene glycol: 31.69; thevalue indicates dielectric constant), EG (ethylene glycol: 38.66), DMF(N,N′-dimethylformamide: 36.71), N,N′-dimethylacetamide: 36.71, DMSO(dimethyl sulfoxide: 46.5), HMPA (hexamethylphosphoric triamide: 29.6),NMP (N-methyl-2-pyrrolidone: 32.3), and the like.

A content of the organic solvent is not limited specifically, but it ispreferably in the range from 0.1 wt % to 10 wt % of a conductiveadhesive.

The moisture-proof reliability of the conductive adhesive is improvedfurther when the conductive adhesive includes an additional materialhaving an action of removing a metal oxide film. Natural oxide filmsthat can be formed on the surfaces of the second particles will lowerthe surface activity of the second particles. Such a natural oxide filmcan be formed on the surface of an electrode as well, and the film willcontrol sacrificial corrosion of the second particles and thus,self-corrosion will be accelerated. Therefore, it is possible to controlself-corrosion of the second particles and accelerate sacrificialcorrosion by adding a material for removing a metal oxide film so as toremove or decrease the oxide film on the surface of the second particles(and on the electrode after packaging). Control of the self-corrosion ofthe second particles is also effective in keeping the moisture-proof fora long time. The metal oxide is not necessarily removed completely, butit can be reduced when compared to a case where the above-mentionedmaterial is not included in the conductive adhesive.

Addition of an activator is helpful to remove or decrease metal oxidefilms on surfaces of the second particles and/or the electrode. Theactivator used here can be an ingredient that will be added to a solderflux; for example, activated rosin, a triol-based compound, and ahalogenated organic compound. In addition, various organic acids,organic acid chlorides, inorganic acids, inorganic metallic acidchlorides or the like having the above-mentioned action can be used.Specific examples of the activator include oleic acid, lactic acid,benzoic acid, o-aminobenzoic acid, m-aminobenzoic acid, p-aminobenzoicacid, glycerol, citric acid, stearic acid, oxalic acid, urea, thiourea,ethylenediamine, diethylenetriamine, hydrazine, glutamic acidhydrochloride, aniline hydrochloride, cetylpyridine bromide, abieticacid, phenylhydrazine hydrochloride, tetrachloronaphthalene,methylhydrazine hydrochloride, dimethylamine hydrochloride, diethylaminehydrochloride, dibutylamine hydrochloride, cyclohexylaminehydrochloride, diethylethanolamine hydrochloride, zinc chloride,stannous chloride, potassium chloride, cuprous chloride, nickelchloride, ammonium chloride, tin bromide, zinc bromide, sodium bromide,ammonium bromide, sodium chloride, and lithium chloride.

Antioxidants also can be used to decrease metal oxide films formed onthe surfaces of the second particles. The antioxidants availableinclude, for example, sulfur-based antioxidant, phosphorus-basedantioxidant, amine-based antioxidant and phenol-based antioxidant; suchas, phenyl salicylate, monoglycol salicylate,2-hydroxy-4-methoxybenzophenon, 2(2′-hydroxy-5′-methylphenyl)benzotriazole, 2-mercaptobenzimidazole, N-salicyloyl-N′-acetylhydrazine,6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline, phenyl-B-naphthylamine,α-naphthylamine, 2,6-di-t-butyl-p-cresol, 2,6-di-t-butyl-phenol,triphenyl phosphite, tridecyl phosphite, trioctadecyl phosphite,trilauryl trithiophosphite, ascorbic acid, glucose, dilaurylthiodipropionate, distearyl thiodipropionate, 2-mercaptobenzimidazole,dilauryl sulphide, propyl gallate, octyl gallate, dodecyl gallate, andB-B′-thiodipropionic acid.

A content of at least one additive selected from the activator and theantioxidant is not limited specifically, but it is preferably in a rangefrom 0.1 wt % to 10 wt % of the conductive adhesive.

In general, the conductive adhesive should include a binder resin. Thebinder resin can be a thermosetting resin such as an epoxy resin, aphenol resin, a urea resin, a melamine resin, a furan resin, anunsaturated polyester resin, a diallyphthalate resin and a siliconeresin. The binder resin can include as well a thermoplastic resin suchas a polyvinyl chloride resin, a vinylidene chloride resin, apolystyrene resin, an ionomer resin, a methylpentene resin, apolyallomer resin, a fluorine resin, a polyamide resin, a polyimideresin, a polyamide imide resin, and polycarbonate. However, since athermoplastic resin will lower the bond strength, the binder resin ispreferably composed of a thermosetting resin.

A content of the binder resin is not limited specifically, but apreferable range is from 5 wt % to 25 wt % of a conductive adhesive.

The conductive adhesive can include further a curing agent, a bondingmodifier, a discoloring inhibitor, a sagging inhibitor, or the like.

The following description is about a second aspect.

A surface of an electrode of an electronic element and/or a circuitboard is modified, and a metal compound coating is formed on the surfacein order to raise the electrode potential. In this case, an especiallypreferred relationship of the standard electrode potentials is asfollows:

(metal compound coating)>(metal particles/first particles)>(silver).

Corrosion in the electrode can be controlled effectively by selectingthe electrode potential to be relatively high. Corrosion in the metalparticles will not progress substantially since the metal particles havea standard electrode potential that is equal to or higher than that ofsilver. There is no need to form metal compound coatings on all of theelectrodes, but such a coating will be formed on an electrode that needsprotection from corrosion, i.e., an electrode having a standardelectrode potential lower than that of the metal particles.

Specifically, the metal compound coating can be modified, for example,by sulfurizing the surface metal of the terminal electrode or bycontacting the surface with an inorganic acid so as to form a metalchloride. The modification method is not limited specifically, but,sulfurization caused by a contact with hydrogen sulfide is a preferredexample.

Preferably, electrical resistivity of the metal compound coating is notmore than 1×10⁻⁴ Ωcm, so that harmful effects for the connectionresistance of the packaging structure can be prevented.

Preferably, the metal compound coating comprises a metal compound thatis substantially insoluble in water. Substantial water insolubilityindicates here that the solubility s (maximum quantity soluble in 100 gof water) is less than about 1×10⁻² g, and solubility product Ksp forwater is less than about 1×10⁻⁵. Both the solubility s and thesolubility product Ksp are based on values at water temperature of 20°C. hereinafter.

The metal compound coating is preferably a coating of a metal sulfide,since many metal sulfides are compounds having water-insolubility andgreat electroconductivity. For example, tin sulfide (SnS) is insolublein water (solubility product: 1×10⁻²⁷) and it has an electricalresistivity of not more than 1×10⁻⁴ Ωcm. Such a metal compound composinga coating can be a metallic salt such as a chromate, an oxalate, aphosphate, or a sulfate; or the metal compound can form a complex.

Since the metal compound coating controls corrosion as well in thiscase, electrodes of the electronic element and of circuit board can bemade of a base metal. There is no limitation on the base metal used inthe electrodes, as the corrosion control demonstrates a desired effectwhen using any of the above-described metals in which galvanic corrosionprogresses easily.

FIG. 1 is a plan view illustrating an example of a packaging structureusing chip parts. This packaging structure is composed bysurface-packaging chip resistors 3, 4, and 5 of a distinct electricstructure on an electrode 2 on a ceramic circuit board 1. A conductiveadhesive layer 6 comprising a conductive adhesive according to thepresent invention is provided on the electrode 2. The electrode 2 andthe chip parts 3-5 are electrically connected to each other via theconductive adhesive layer 6. Alternatively, a metal compound coating isformed on the electrode 2 on the circuit board and/or on electrodes ofthe chip parts. The chip resistors can be replaced by other parts suchas chip capacitors.

FIG. 3A is a perspective view to illustrate a QFP as an example of alead element. Plural leads 12 protruding from side faces of the device11 of the lead element extend downwards in a curved state. As shown inFIG. 3B, this QFP is joined to a land 14 of a substrate 15 via aconductive adhesive 13.

EXAMPLES

The present invention will be described below in detail with referenceto examples, however, the present invention is not limited to theexamples.

In the following Examples 1-12 and Comparative Examples 1-2, change inelectrical resistance was measured by using a specimen shown in FIG. 2.This specimen comprises a substrate 7 on which electrodes 8 and 9 areformed with a space of 30 mm therebetween. The electrodes 8 and 9 havesurfaces of a SnPb alloy (Sn₉₀Pb₁₀ alloy). A conductive adhesive wasmanufactured by preparing metal particles A comprising 7 wt % bisphenolF-type epoxy resin (liquid), 2 wt % additives (a dispersant, a bondingmodifier etc.) and 89 wt % Ag, adding 2 wt % of predetermined metalparticles B, and kneading the mixture by using a three-roll typeapparatus. In addition to that, an organic solvent, an activator, and anantioxidant were added to some samples. For such samples, amount of thebisphenol F-type epoxy resin was decreased by the same amount of theadditional ingredients.

Here, the metal particles A are substantially spherical and the averagediameter is 2-15 μm.

Subsequently, a conductive adhesive layer 10 was formed to bridge theseelectrodes by screen printing. Furthermore, the conductive adhesivelayer was heated to cure in an oven for 30 minutes at a temperature of150° C. The thus prepared specimen was subject to a humidity test bybeing left for 1000 hours in a thermo-hygrostat bath kept at 85° C. witha relative humidity of 85% in order to measure the electrical resistancebetween the electrodes 8 and 9 before and after the test (an initialresistance value and a measurement value after the test).

(Example 1)

The metal particles B used here were Ni particles (substantiallyspherical and the diameter is 3-7 μm, or the average diameter is 5 μm).

The standard electrode potential meets the relationship ofNi(−0.25V)<Sn(−0.14V)<Pb(−0.13V)<Ag(+0.80V). Therefore, the metalparticles B have a potential lower than that of the SnPb alloy.

(Example 2)

The metal particle B used here were carbon steel particles (sphericaland the average diameter is 5 μm, the carbon content is 5 wt %).

The standard electrode potential meets the relationship of C(−0.76V)<Pb(−0.50V)<Sn(−0.42V)<Ag(−0.13V) in a deionized water including 3 wt %NaCl. Similar to Example 1, the relationship between the C, Pb, Sn andAg concerning the standard electrode potential measured in the deionizedwater is as described above.

(Examples 3-12)

The metal particles B used here were Zn particles (spherical and theaverage diameter is 5 μm).

Under the above-mentioned measurement condition where a chloride ionsexist, the standard electrode potential meets a relationship ofZn(−1.03V)<Pb(−0.50V)<Sn(−0.42V)<Ag(−0.13V). Zn will form an oxide moreeasily than Ni or C does. For a measurement concerning the deionizedwater, relationship of the standard electrode potentials between Zn, Pb,Sn and Ag is same as the relationship mentioned above.

In Examples 4-12, an organic solvent and/or an activator/antioxidantwere added further in an appropriate manner.

(Comparative Example)

Measurement was performed as in the above Examples without adding themetal particles B.

The following Table 1 shows the thus obtained results.

TABLE 1 Metal particles NaCl Solvent Activator/antioxidant Initialresistance Resistance after test B (wt %) (ppm) (wt %) (wt %) (mΩ) (mΩ)Ex. 1 Ni 2 5 — — 13 29 Ex. 2 C steel 2 5 — — 13 21 Ex. 3 Zn 2 5 — — 1318 Ex. 4 Zn 2 5 BC 2 — 12 16 Ex. 5 Zn 2 5 DEG 2 — 12 15 Ex. 6 Zn 2 5 DEG2 L(+)-ascorbic acid 0.5 11 11 Ex. 7 Zn 2 5 DEG 2 D(+)-glucose 0.5 11 11Ex. 8 Zn 2 5 DEG 2 Oleic acid 0.5 11 11 Ex. 9 Zn 2 5 DEG 2 Glycerol 0.511 11 Ex. 10 Zn 2 5 DEG 2 Zinc chloride 0.5 11 11 Ex. 11 Zn 2 5 DEG 2Phenyl salicylate 0.5 11 11 Ex. 12 Zn 2 5 DEG 2 Octyl gallate 0.5 11 11Com. Ex. 1 — 5 — — 12 326  BC: diethylene glycol monobutyl ether, DEG:diethylene glycol Ex.: Example Com. Ex.: Comparative Example

Detected sodium chloride is considered to have been included in thebisphenol F-type epoxy resin. Resistance value in Example 1 was slightlyhigher than in other examples, since Ni has a standard electrodepotential higher than that of Pb under the above-mentioned measurementcondition having electrolytic ions. The results in Table 1 demonstratethat addition of an organic solvent, an activator/antioxidant iseffective to prevent the resistance value from being lowered in theinitial stage and being raised subsequently.

(Examples 13-24)

Actual chip element structures were manufactured by using the conductiveadhesives of Examples 1-12.

Similar to the structure shown in FIG. 1, an electrode of SnPb-plated Cuwas formed on a surface of a ceramic circuit board (30×60 mm, 1.6 mm inthickness). Using any of the above-mentioned conductive adhesives, thiselectrode was packaged with a 0Ω chip resistor (3216 size, SnPb plated),a chip coil (8 mmφ in diameter, 4 mm in height, SnPb-plated), and a chipcapacitor (3216 size, SnPb-plated). The conductive adhesives wereapplied and cured in the same manner as described in the above Examples.

In Examples 13-24, the conductive adhesives of Examples 1-12 are usedrespectively.

(Example 25)

In this example, a chip element packaging structure was manufactured asin Examples 13-24 by using a conventional conductive adhesive describedin Comparative Example 1. In Example 25, the surface metal of theelectrode on the ceramic circuit board was sulfurized. Specifically, aceramic circuit board was introduced in a closed tank 0.34 m³ incapacity, being kept at a temperature of 40° C. and at a relativehumidity of 90%, and fed with hydrogen sulfide for 24 hours so that thehydrogen sulfide concentration in the tank became 3 ppm before theceramic circuit board was applied with the conductive adhesive. Thestandard electrode potential meets a relationship ofAg(0.80V)<SnS(0.87V)<PbS(0.93V).

SnS and PbS are substantially insoluble in water (a solubility productKsp of SnS is 1×10⁻²⁷, and a solubility s of PbS is 1×10⁻³ g/100 gwater), and they have a high conductivity, i.e., the electricalresistance is not more than 1×10⁻⁴ Ωcm.

(Comparative Example 2)

A chip element packaging structure similar to those of Examples 13-24was manufactured by using a conventional conductive adhesive ofComparative Example 1. In this comparative example, the electrode on thecircuit board was not surface-treated.

The thus obtained chip element packaging structure was kept for 1000hours in a thermo-hygrostat bath maintaining a relative humidity of 85%at 85° C., and the series resistance of three parts was measured beforeand after the treatment. The results are shown in Table 2.

TABLE 2 (Resistance value: Ω) Initial Measuring Conductive Electrodesurface resistance value Adhesive treatment value after test Example 13Example 1 — 2.5 3.6 Example 14 Example 2 — 2.5 3.1 Example 15 Example 3— 2.5 2.8 Example 16 Example 4 — 2.4 3.5 Example 17 Example 5 — 2.4 3.0Example 18 Example 6 — 2.3 2.5 Example 19 Example 7 — 2.3 2.5 Example 20Example 8 — 2.3 2.5 Example 21 Example 9 — 2.3 2.5 Example 22 Example 10— 2.3 2.5 Example 23 Example 11 — 2.3 2.5 Example 24 Example 12 — 2.32.5 Example 25 Com. Ex 1 sulfuration 2.5 2.6 (SnS, PbS) Com. Ex. 2 Com.Ex 1 — 2.5 251.2

(Examples 26-36)

Lead element structures were manufactured as in the above-mentioned chipelement structures by using the respective conductive adhesives, and thestructures were subjected to a humidity test.

Each lead element structure was configured as shown in FIG. 3 by forminga land electrode of SnPb-plated Cu on the surface of the circuit boardand by packaging 25 QFPs with a package size of 15×15 mm per lead byusing any of the above-mentioned conductive adhesives. Spacing betweenthe lead terminals (12 in FIG. 3) was 0.5 mm. The resistance value wasmeasured for all of the spacing between adjacent lead terminals beingconnected electrically by a daisy chain through the device, and theaverage value was determined as the resistance value.

The conductive adhesives and resistance values before and after thehumidity test are shown in Table 3.

TABLE 3 Metal particles NaCl Solvent Activator/antioxidant Initialresistance Resistance after test B (wt %) (ppm) (wt %) (wt %) (mΩ) (mΩ)Ex. 26 Zn 2 5 — — 53 79 Ex. 27 Zn 2.5 5 — — 53 58 Ex. 28 Zn 3 5 — — 5356 Ex. 29 Zn 3 5 DEG 2 — 53 55 Ex. 30 Zn 3 5 DEG 2 L(+)-ascorbic acid0.5 52 52 Ex. 31 Zn 3 5 DEG 2 D(+)-glucose 0.5 52 52 Ex. 32 Zn 2 5 DEG 2oleic acid 0.5 52 52 Ex. 33 Zn 2 5 DEG 2 glycerol 0.5 52 52 Ex. 34 Zn 25 DEG 2 zinc chloride 0.5 52 52 Ex. 35 Zn 2 5 DEG 2 phenyl salicylate0.5 52 52 Ex. 36 Zn 2 5 DEG 2 octyl gallate 0.5 52 52 Ex.: Example

As shown in Table 3, an effect of the added second particles (metalparticles B) in packaging a lead element became apparent when thecontent exceeded 2 wt %, and the effect was stabilized when the contentwas 3 wt % or more.

In Example 15, the cross section of the conductive adhesive after thetest was analyzed by SIMS (secondary ion composition analysis method).As indicated in FIGS. 4A and 4B, areas with a high Zn concentration andareas with a high O concentration correspond well with each other.Narrower hatching pitches in FIGS. 4 and 5 indicate that theconcentrations of the detected elements were high. In the Zn particlesof the conductive adhesive, oxygen atoms were dispersed in asubstantially uniform state after the humidity test. The Zn particlesare considered to have been changed into either an oxide or a hydroxideas a whole due to the sacrificial corrosion.

In Comparative Example 2, the cross section in the vicinity of theinterface between the conductive adhesive and the element electrodeafter the test was analyzed by SIMS. As shown in FIGS. 5A and 5B, theoxygen concentration was raised slightly where the Sn concentration washigh. On the other hand, oxygen concentration in the conductive adhesivewas lower than that on the electrode surface (FIG. 5B).

The above Examples relate to chip element packaging structures and leadelement packaging structures. The present invention is not limitedthereto but it can be used for packaging various parts such as packagingparts including CSP (chip scale package), BGA (ball grid array), chipparts/lead parts including electrolytic capacitors, diodes and switches,and IC bare packages.

As mentioned above, the present invention provides a conductive adhesiveand a packaging structure with improved moisture-proof reliability.Especially, since the present invention can improve the moisture-proofwhile using a multipurpose base metal electrode, it can increase rangeof uses for a conductive adhesive and a packaging structure using thesame.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Theembodiments disclosed in this application are to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims rather than by the foregoingdescription, all changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

What is claimed is:
 1. A conductive adhesive for connecting electricallyan electronic element and a substrate, comprising first particles forsecuring electric conductivity, second particles having a standardelectrode potential lower than a standard electrode potential of thefirst particles, and 1 ppm to 10000 ppm of electrolytic ions.
 2. Theconductive adhesive according to claim 1, wherein the second particlescomprise at least one element selected from Zn, Fe, Mg, Cu, V, Ca andBe.
 3. The conductive adhesive according to claim 2, wherein the secondparticles comprise Zn.
 4. The conductive adhesive according to claim 1,wherein a content of the first particles ranges from 70 wt % to 95 wt %.5. The conductive adhesive according to claim 1, wherein a content ofthe second particles is not less than 0.5 wt % and not more than 10 wt%.
 6. The conductive adhesive according to claim 5, wherein a content ofthe second particles is more than 2 wt %.
 7. The conductive adhesiveaccording to claim 1, wherein the standard electrode potential of thesecond particles is lower than that of Sn.
 8. The conductive adhesiveaccording to claim 1, wherein the electrolytic ions comprise chlorideions.
 9. The conductive adhesive according to claim 1, furthercomprising a binder resin.
 10. The conductive adhesive according toclaim 9, wherein the binder resin is a thermosetting resin.
 11. Theconductive adhesive according to claim 1, further comprising an organicsolvent, wherein the content of the organic solvent is in the range of0.1 wt % to 10 wt %.
 12. The conductive adhesive according to claim 11,wherein the organic solvent is a polar solvent having a dielectricconstant of at least
 15. 13. The conductive adhesive according to claim1, further comprising a material for removing a metal oxide film,wherein the material removes or decreases metal oxide films on thesurfaces of the second particles.
 14. The conductive adhesive accordingto claim 1, further comprising at least one agent selected from anactivator and an antioxidant.
 15. A conductive adhesive for connectingelectrically an electronic element and a substrate, comprising firstparticles for securing electric conductivity, second particles having astandard electrode potential lower than a standard electrode potentialof the first particles, and a polar solvent having a dielectric constantof at least
 15. 16. The conductive adhesive according to claim 15,wherein the second particles comprise at least one element selected fromZn, Fe, Mg, Cu, V, Ca and Be.
 17. The conductive adhesive according toclaim 16, wherein the second particles comprise Zn.
 18. The conductiveadhesive according to claim 15, wherein a content of the first particlesranges from 70 wt % to 95 wt %.
 19. The conductive adhesive according toclaim 15, wherein a content of the second particles is not less than 0.5wt % and not more than 10 wt %.
 20. The conductive adhesive according toclaim 19, wherein a content of the second particles is more than 2 wt %.21. The conductive adhesive according to claim 15, wherein the standardelectrode potential of the second particles is lower than that of Sn.22. The conductive adhesive according to claim 15, further comprising abinder resin.
 23. The conductive adhesive according to claim 22, whereinthe binder resin is a thermosetting resin.
 24. The conductive adhesiveaccording to claim 15, further comprising a material for removing ametal oxide film, wherein the material removes or decreases metal oxidefilms on the surfaces of the second particles.
 25. The conductiveadhesive according to claim 15, further comprising at least one agentselected from an activator and an antioxidant.